Simplify the following expression: $k = \dfrac{r}{10} - \dfrac{r}{2}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $10$ and $2$ $\lcm(10, 2) = 10$ $ k = \dfrac{1}{1} \cdot \dfrac{r}{10} - \dfrac{5}{5} \cdot \dfrac{r}{2} $ $k = \dfrac{r}{10} - \dfrac{5r}{10}$ $k = \dfrac{r -5r}{10}$ $k = \dfrac{-4r}{10}$ Simplify the expression by dividing the numerator and denominator by 2: $k = \dfrac{-2r}{5}$